1. Technical Field
In the course of electrical signal transmission, certain unwanted and undesirable effects take place. One is attenuation, which reduces the signal strength; more serious, however, are distortion, interference, and noise, which appear as alterations of the signal shape. Broadly speaking, any unintended signal perturbation may be classified as "noise", and it is sometimes difficult to distinguish the various offenders in a contaminated signal. Distortion is signal alteration due to imperfect response of the system to the desired signal itself. Unlike noise and interference, distortion disappears when the signal is turned off. Interference is contamination by extraneous signals, usually manmade, of a form similar to the desired signal. The problem is particularly common in broadcasting, where two or more signals may be picked up at the same time by the receiver. Noise means the random and unpredictable electric signals which come from natural causes, both internal and external to the system. When such random variations are added to an information-bearing signal, the information may be partially masked or totally obliterated. Of course the same can be said for interference and distortion; what makes noise unique is that it can never be completely eliminated, even in theory. Noneliminable noise poses one of the basic problems of electrical communication.
The present invention relates to precise measurement of phase angles between harmonic components of two general distorted signals embedded in measurement noise. The invented phase measurement method is simple and is implemented in the design of a new instrument capable of measuring: amplitude of harmonic components of the two input signals and phase shifts between the corresponding harmonic components of the input signals. If the input signals represent voltage and current in a network then the new instrument can measure precisely voltage, current, load impedance, the active power and reactive power of the network. The invention relates also to harmonic distortion measurement and filtering of preselected harmonic components. The apparatus design will be described, which can be used for telecommunication applications such as to find the percentage modulation in transmitting communication units and demodulation of the received signals or to find digital time-of-flight measurement from ultrasonic sensors to measure object distance in air and flow velocity.
2. Description of the Prior Art
Conventionally, the phase of a signal is measured by using what may be termed the "null technique." The null technique involves the shifting of the phase of the test signal whose phase is being measured until a device such as phase detector indicates that signal is in quadrature with a reference signal. Such a point is indicated by the null output of the detector. The amount of phase shift required to reach null indicates the phase shift of the input test signal.
Sheftleman, U.S Pat. No. 3,249,868, addresses phase measurement of noise-contaminated signal. Generally, the apparatus of Sheftleman's invention includes a pair of phase detectors which are driven in quadrature by separate reference voltages. The ratio of the average outputs of phase detectors is measured to provide an indication of phase displacement. The two phase detectors are supplied with the same input signal. More specifically, the average outputs from the pair of phase detectors are compared in a threshold device to indicate their ratio. This ratio is a function of the phase displacement of the input signal applied to the two phase detectors. There are two principal disadvantages in the threshold circuit technique. First, the operator needs to compute the ratio of the two meter readings, and second, the length of time required for one of the thresholds to be reached depends on the amount of noise present with the signal. The circuit adds two different noise levels in each phase detector circuit due to the amount of noise present in the reference signals.
Kummerer, U.S. Pat. No. 1,708,544, described a method and arrangement for measuring frequency and frequency variations in alternating currents. It also embraces a method and means for the comparison of the phase relation of two alternating currents. A phase change of detuning of an oscillation circuit with relation to an excitation source of oscillation is utilized. This is done in combination with a Wheatstone bridge arrangement which is built such that the resistances of the different bridge arms or branches are influenced by the phase-displaced currents of an excitation circuit and of an excited circuit. The supply of current to the bridge is such that in the presence of a phase displacement of the current of exactly 90 degrees (quadrature) as occurring at resonance, the effective values of the currents flowing in the bridge arms are equal. The bridge arms contain resistances which change with heating. The bridge furthermore is fed from a direct current source. The zero position of the output is influenced by noise. Harmonic components of the waveforms are not considered but they affect the change of the resistances of the arms.
Adorjan, U.S. Pat. No. 2,213,099, developed a distortion indicator for electrical amplifying systems, particularly thermionic amplifying systems. The test signal to be amplified is applied to the terminals of a thermionic amplifier, the output terminals of which feed a load circuit. A fraction of the input voltage is obtained by means of an attenuator which, after passing through a filter, is applied to a bridge rectifier circuit. The uni-directional output of the rectifier is then applied to two wound moving coils rigidly secured together at right angles and disposed in a magnetic field produced by a permanent magnet. By suitable adjustment of the attenuators, the currents in the coils may be adjusted so that the torque-producing effects of the two currents exactly balance each other. Provided that the ratio of the output to input voltages remains the same, the ratio of the rectified voltages will be unaltered and no deflection of the pointer occur. If, however, there should be a change of ratio of output to input voltages, then there will be a deflection of the pointer and this indicates the amount of distortion being produced by the amplifier. This method is limited by the sensitivity with the moving coils system and the adjustments which can be made in the attenuators. Further, this method cannot give the distortion due to preselected harmonics.
Diehl, U.S. Pat. No. 2,986,700 developed a circuit for determining whether the phases of carrier voltages are in phase quadrature. The circuit includes a balanced modulator to which a pair of carrier voltages in quadrature phased relationship are applied and from which a small amount of second harmonics of the applied carrier voltages is obtained. The circuit also comprises a single tuned amplifier selectively responsive to amplifying the second harmonics of the applied carrier voltages. This circuit cannot be used except for carrier frequency signals with phase difference of 90 degrees.
Ule, U.S. Pat. No. 3,005,151, invented a phase measurement method for a sinusoidal signal passing through a network under test. This method relies on generating a sinusoidal voltage and an alternating signal having substantially the same half-periods, and impressing the sinusoidal voltage on the network to produce a signal at the output of the network. The generation of the alternating signal is stopped at a time when the amplitudes of the alternating and output signals bear a predetermined relationship with respect to each other, and restarted at a time when the amplitudes of the signals cease to bear the predetermined relationship with respect to each other. The length of time that the generation of the alternating signal is stopped is then used to measure the signal's phase. The major disadvantage of this method is that it does not discriminate between the noise effects or the distortion effects on the signal.
Kirsten, U.S. Pat. No. 3,016,475, relates to an electronic counting circuit and more particularly to a system for indicating the net accumulated phase difference between two continuing input signals. The circuit registers count either additive or subtractive depending upon the direction of phase change. The major disadvantage is that the two signals must have the same frequency or cycle's time and that they must be free from distortion and measurement noise.
MacMillan, U.S. Pat. No. 3,019,390, provides a system for measuring the phase of an unknown signal in which a reference signal source supplies AC sinusoidal voltage having a controllable phase shift between zero and 360 degrees. The system includes a DC comparator and chopper amplifier, phase rectifier and filter, and sine and cosine potentiometers. The mode of operation provides an automatic and continuous control of the input amplifier, wherein the two voltages are applied to the comparator and chopper amplifier. Initially, for a specific frequency, the phase of one input voltage is adjusted to zero while the other is set to 90 degrees. The input voltages are precisely equal in amplitude. Each of the two rectifier-filters provides a DC voltage to cause a change in the charge of a coupling capacitor. The system automatically and continuously controls an amplifier so that an AC output voltage results which is always precisely equal in amplitude and 90 degrees phase shift to the AC input voltage, regardless of frequency variation. This method requires a special reference signal and is subject to the same disadvantages as of the Ule and Kristen inventions.
King, U.S. Pat. No. 3,281,846, provides an apparatus for comparing the amplitudes of two alternating current signals differing from one another in frequency. A half-wave or balanced modulator type of phase discriminator, is used to generate two output signals to feed a balanced network. A meter will move to the left or right of center depending upon the phase of the signal. This method can be used for measuring the ratio between any two harmonic components. In order to accomplish this, the distorted input signal has to pass through two parallel branches of a bandpass filter tuned to one of the harmonic frequencies. King's invention cannot be used for measuring the phase differences of harmonic components or can cancel noise effects.
Gerst et al, U.S. Pat. No. 4,277,748, provides an angle digitizer from a signal representing the sine and cosine of the angle to be digitized. The voltages are fed to orthogonal pairs of input terminals between which are a set of equivalued resistors forming a resistor ring. At the junction of each pair of resistors is an output terminal. Each pair of output terminals is connected to the input terminals of a signal amplitude comparator which compares the magnitude of the signals at its inputs to give a digital signal in accordance with which is greater. For each comparator, the output terminals of the resistor ring are selected such that the signals are in a direct antiphase relationship. This invention is limited in application to pure sine or cosine waveforms not embedded in noise.
C. Reynolds described three techniques for measuring phase-noise sidebands in his article, "Measure phase noise in one of three ways, each of which has some advantages. Quadrature phase detection avoids dynamic-range limits" 25 Electronic Design, 106-108, Feb. 15, 1977, using frequency analyzers with dynamic ranges of 160 dB and 1-Hz bandwidths in the gigahertz region. The techniques used are rf-spectrum measurement, frequency discrimination and quadrature-phase detection. To improve frequency resolution, the carrier frequency component is eliminated. To improve resolution in a phase-noise measurement, a mixer and low-pass filter shift signal frequencies down. In practice, phase-noise analysis often covers a frequency range greater than that of a single selective analyzer. Consequently, two analyzers can be used. Two units covering the range of 5 Hz to 13 MHz, for example, can test a 10 MHz synthesized source, using a phase-locked system. To improve noise measurements, an automatic spectrum analyzer may be selected with a calculator controller covering the range from 100 Hz to 1 MHz. The programmable power of such a system allows the user to select points that avoid discrete signals and thus quickly determine a phase-noise-sideband envelope over a wide frequency range. The key to this capability is a programmable synthesizer combined with a tracking analyzer that features digital readout and output, rather than a built-in cathode ray tube (CRT). The automatic analyzer's internal structure is similar to most spectrum analyzers. The programmable calculator, through software written by the operator, controls both analyzer and synthesizer over a bidirectional interface. The calculator manipulates data received from the analyzer and plots the normalized, corrected results on an optional digital plotter. Although these three methods can be used for measuring the amplitude of harmonic components, they cannot be used for measuring the phase shifts of harmonic components.
None of the above inventions or methods above describes a method for the measurement of the amplitudes of the harmonic components of two input signals as well as the phase shift between the harmonic components of the two input signals.
Accordingly, it is a primary objective of the invention to present an instrument based on a new design capable of measuring and analyzing multi- distorted signals subject to measurement noise and extract useful signal characteristics for any two of the input signals.
Another objective of the invention is to provide an apparatus for processing two input signals: first signal and second signal wherein the input signals may represent the voltage and the current in a power network and wherein the waveforms of the input signals are assumed to be distorted and contain a measurement noise component.
Another objective of the invention is to present an instrument capable of measuring certain harmonic components of first test signal or a sum of certain preselected harmonics.
Another objective of the invention is to present an instrument capable of measuring certain harmonic components of second test signal or a sum of certain preselected harmonics.
Another objective of the invention is to present an instrument capable of measuring the phase angle between the corresponding harmonic components of two input test signals.
Another objective of the invention is to present an instrument capable of measuring the active or reactive power in a power network wherein the two input signals represent the applied input voltage and the current in the network.
Another objective of the invention is to present an instrument capable of measuring the total harmonic distortion for any two of the input signals in analog form or in digitized form.
Another objective of the invention is to present an instrument capable of providing an output having a filtered waveform from any of the input signals by filtering out DC components of signal or preselected harmonic components.
Another objective of the invention is to present an instrument capable of finding the time difference between emitted wave and echoes, using cross correlation algorithms involved in the design of the apparatus, from the inspection of the peak of the cross correlation function of emitted wave and echoes.
Another objective of the invention is measuring load impedance being determined from the ratio of voltage and current across the impedance at a given harmonic frequency thereof by applying voltage and current signals to the inputs of the apparatus.
Another objective of the invention is measuring the impedance of a multi-phase network as well as the comparison of each phase impedance to detect any unbalance in the multi-phase network, or identify fault modes in the multi-phase network.
A final objective of the invention is the use of the apparatus for telecommunication applications such as to find percentage modulation in transmitting communication units and demodulation of the received signals from the amplitudes and phase shifts of harmonic components of the test signals.